Update /help/privacy

https://github.com/mysociety/whatdotheyknow-theme/issues/132

    From: Tom Steinberg <tom@mysociety.org>
    Date: Fri, 7 Mar 2014 20:42:01 +0000
    Subject: Please put this into the sprint on Monday :)
    To: gareth@mysociety.org, Mark Longair <mark@mysociety.org>

    https://github.com/mysociety/whatdotheyknow-theme/issues/132

    It's the output of a very long policy discussion with the WDTK volunteers.

    thanks,

    Tom